Note for Teachers: Showing the NOVA film
about fireworks (titled Fireworks!) before or after this
lab activity may enhance the learning it is meant to
produce.
Also, I have produced a video suitable as an introduction
to this lab. It may also be viewed by students who were
absent.
Objective
In this lab students will learn about atomic energy levels,
emission spectroscopy, and flame tests for element
identification.
Overview
Students will observe small samples of chloride salts of
different metals. These will be placed into a flame in
order to observe the colors produced. These colors come
from the excitation of electrons which then resume their
ground states by emitting light of very specific colors.
Background
The electrons in an atom occupy different energy levels.
When all of the electrons are at the lowest possible energy
level they are said to be in the ground
state. In this state electrons are as close to the atom’s nucleus as they are allowed to be. When they are there, they have their lowest possible potential energy. Just as a ball sitting on the ground has its lowest potential energy, electrons which are close to the nucleus are similarly low in potential energy.
Electrons do not always stay in the ground
state. Sometimes they can be promoted to an
electron shell with a higher potential energy. This can happen in two ways. First, the
electron can absorb a photon of just the right amount of
energy to move it from one quantum shell to another.
Second, when atoms are heated in a flame or energized with electricity
their electrons can gain energy. This promotes them to the
higher-energy shell. When an electron is in a higher-energy
shell it is said to be in an excited
state. Excited states are situation where one or more electrons within an atom are at a higher potential energy than they would be in the ground state. There are many possible excited states for atoms.
Electrons in excited states do not stay in them for very
long. When electrons lose their energy they do so by
emitting light. In this way, excess potential energy is transformed into the energy of light. Each time a single electron drops from higher to lower potential energy, a particle called a photon is produced. Photons are particles with
energy but no mass. Their energy is inversely proportional
to the wavelength of the light (this can be expressed in the
proportion, E = hc/λ, where λ means the wavelength). The photons emitted precisely match
the quantum energy difference between the excited state and
the ground state.
A salt is a type of compound that include a
metal ion and a non-metal ion. Sodium chloride
(NaCl) is the most familiar
example of a salt but others include calcium
chloride(CaCl2) and
copper(II) chloride (CuCl2). These compounds look very much the same to the unaided eye. For example both sodium chloride and calcium chloride are colorless crystals and as a fine powder they are white. In order to identify which salt you have, you have to do a test on the material. A traditional method for identifying elements in compounds is called a flame test.
In flame tests salts
that are dissolved in water are evaporated using a hot
flame. In the flame the atoms become excited and
produce a characteristic color. This happens when the atoms’ excited electrons fall back down to the ground state. The color we see depends on the
difference in energy between the excited and ground state.
For some atoms this is a large difference and for others
it is a smaller difference. The color can be used to tell if the energy difference is large or small. The purple end of the spectrum has the smallest wavelength and so the energy of that light is the largest. The red end of the spectrum has the largest wavelength and so that color represents the smallest energy changes. Remember the order of colors in a rainbow is ROYGBIV (red orange yellow green blue indigo violet). Energy changes can be judged against the spectrum because as you go from red to violet the energy change that made the color gets larger. Different atoms have different possible excited states, which vary in their distance from the atom’s ground state. Each element has a unique set of possible excited states. Because of this it is possible to identify an element by doing a flame test and recording the observed color. Every element has a unique flame test color.
It is a traditional art of the
chemistry laboratory to use these colors to identify
specimens of compounds that contain unknown metals. This is no longer a common laboratory technique though highly advanced technologies use the same production of light to identify elements and to measure how much is present. The production of light in a flame is still used outside the lab, though. It is put
to use by practitioners of the art of fireworks
manufacture. By including different metal salts, or
mixtures of metal salts, in the exploding shell of a
firework, these artists can produce beautiful displays in
nearly all the colors of the rainbow.
Color
Representative
Wavelength (nm)
Wavelength
Region (nm)
Violet
420
400 - 440
Blue
455
440 - 470
Blue-green
480
470 - 490
Green
525
490 - 560
Yellow-green
565
560 - 570
Yellow
580
570 - 585
Orange
620
585 - 630
Red
660
630 - 700
Light is a kind of wave, an electromagnetic wave. Our
ability to perceive color depends on the different
wavelengths of light. If you observe a rainbow or use a
prism to split white light then you are looking at how our
eyes and brain work together to give us the sensation of
the colors of the rainbow. Because we see different ranges
of the visible spectrum as different colors it’s
possible to roughly assign a color we see to a particular
wavelength of light. It is not a perfect match, but
it’s a good start and is good enough for now. Take a
look at the chart at right. It details the colors people in
our culture usually assign to the rainbow and gives each
color a range of wavelengths in units of nanometers. A
nanometer (nm) is a billionth of a meter (1 m = 1 ×
109 nm). Each color can be assigned a
‘representative wavelength’. You will use these
to identify colors in this activity.
Materials
10 small beakers per lab table
one for distilled water
9 for the samples and unknowns
1 inoculation loop
Or a set of cottom swabs
1 Bunsen burner
sharpie for labeling
matches
distilled water
a series of metal chloride solutions such as
CaCl2,
CuCl2,
LiCl,
KCl, NaCl,
CsCl, and SrCl2
(these will be provided in dropper bottles)
2 unknown metal chlorides
Safety
Wear goggles or risk sitting out the lab
Treat all chemicals in this lab as toxic. Do not touch
any of them with your bare hands.
Wash well with water immediately if you touch chemicals
accidentally
Use caution with the burner
Do not leave burner unattended
Place burner near middle of lab bench
Tie back long hair
Do not wear baggy clothing in the lab
Hot objects look like cold objects: be cautious!
CuCl2 (Copper(II)
Chloride) is highly toxic by ingestion; avoid contact
with eyes, skin and mucus membranes.
LiCl (Lithium Chloride) is
moderately toxic by ingestion; avoid contact with eyes,
skin and mucus membranes.
Wash your hands with soap and water after you complete
the day’s lab work, even if you didn’t touch
any chemicals directly
Procedure
Remember to record your observations in your lab notebook
or on a piece of paper in your binder before you leave
class. When making observations be sure to use all of
your senses except taste. Never taste anything in the
chemistry lab. Chances are good you will regret it if you
do.
You will share a set of metal salt solutions with the
people at your lab station. At least four people should
share a set to reduce the number of beakers to be washed
and the amount of chemical waste. Label all of your
beakers. Label one for the water rinse.
Either 50-mL or 100-mL
beakers will be fine.
Collect a small sample (a few drops) of each of the
known metal salt solutions which your teacher has provided
and carry them all to your lab bench.
Obtain an inoculation loop for your group.
Obtain 10 - 20 mL of distilled water
in your labeled beaker.
Each group member must record information in a neat
table with the following columns. Make this table before
you even turn on the gas.
Ionic Compound Formula
Cation (positive ion)
Anion (negative ion)
Color Observed
Representative Wavelength (nm)
Element Responsible for Color
Do not fill in the final column: “Element Responsible for Color”. You will fill in this column as part of the post-lab questions. To help identify ions, you may use your Ions Reference Sheet. Choose whatever color is closest from the table of representative wavelengths in the introduction of this lab.
Clean the inoculation loop by swirling it gently in the
distilled water.
Then, once you light the burner,
heat the loop until it glows red hot. This step removes any ions
clinging to the loop.
Light and adjust your Bunsen burner. Be sure to clean
your loop carefully. Do not leave the loop in the
flame too long as it can cause the loop to degrade and
break.
To do a flame test with each metal salt get a film of
the solution of a salt inside the loop and bring it into
the hottest part of the flame. If this produces poor color
then try the edge of the burner flame. Repeat the dip into
the salt solution as often as necessary to see the flame
test color. Be sure not to over-heat the loop.
Carefully note the color of each metal salt when it is
put in the flame. Use the chart on the previous page to
estimate the approximate wavelength of the color you see.
Use the Representative Wavelength values. Record all data
in the table you made earlier.
Clean the inoculation loop using distilled water
and heat each time you change from one metal
salt to another. Failing to do so will result in mixed
flame test colors.
Again, do not over heat the loop.
Your teacher has prepared two solutions with two of the
metal salts. They are labeled Unknown 1 and Unknown 2. Test these in order to determine what they are. The unknowns each correspond to one of the salts you have observed. By comparing the unknowns with samples of a known identity you will be able to identify them.
Clean out the beaker using the method recommended by
your instructor (hazardous wastes must be disposed of
properly). Usually, all leftover solutions will be
collected in designated waste containers for hazardous
waste disposal.
Wash all equipment carefully and thoroughly using the
tub of soapy water provided. By gently scrubbing the
beakers with a brush and the soapy water you will be able
to wash off the labels you put on them. Please do so!
Grading
For this lab you must turn in the following items:
Your data table recording flame test colors and
wavelengths
Answers to the following questions
A one-page essay about how fireworks are made,
concentrating on how they produce different colors but
including details of their basic construction and operation
Questions
According to the introduction on this lab handout, what is the chemical definition of a salt?
According to the introduction on this lab handout, what is the ground state of an atom?
According to the introduction on this lab handout, what is an excited state of an atom?
Based on your reading of the introduction, what has to happen within an atom (that is, what happens to the electrons in an atom) before it can produce light?
Based on your reading of the introduction, why do atoms need to be heated in a hot flame or subjected to high-voltage electricity before they will produce any light?
Based on your reading of the introduction, which color represents a larger energy change within an atom: blue or orange? Explain.
Determine the element that is responsible for the color of each flame test by considering the following questions. Answer each with appropriately formatted chemical formulas and complete sentences as necessary.
What are the names and formulas of all of the compounds that contain the chloride ion (Cl–)?
Do any compounds containing the chloride ion (Cl–) have the same color? Explain.
What are the names and formulas of all of the compounds that contain the sodium ion (Na+)?
Do any compounds containing the sodium ion (Na+) have the same color? Explain.
What are the names and formulas of all of the compounds that contain the copper ion (Cu2+)?
Do any compounds containing the copper ion (Cu2+) have the same color? Explain.
Is it the cation or the anion that is responsible for the color of a flame test?
After completing the questions above, fill in the final column on your table.